1. Field of the Invention
This invention relates to a catalyst for effectively and economically reducing to innocuous forms the nitric oxide (NO), nitrogen dioxide (NO.sub.2) and the like (hereinafter referred to collectively as nitrogen oxides or NOx) contained in exhaust gases from oil- and coal-fired boilers, combustion furnaces associated with various chemical plants, iron foundries, and internal combustion engines, such as, diesel engines and turbines.
More particularly, the invention concerns a catalyst which, when exposed to so-called "dirty" exhaust gases containing in addition to NOx, sulfur oxides (SOx), soot and dust which tend to poison ordinary catalysts, converts the NOx into harmless N.sub.2 and H.sub.2 O by catalytic reduction with the aid of ammonia (NH.sub.3) used as a reducing agent, the catalyst performing its action efficiently without being affected by the otherwise toxic ingredients, and at low cost without the danger of secondary pollution.
2. Description of the Prior Art
Removal of NOx from waste gases is carried out in a variety of ways; for example, by solidification and collection, adsorption, oxidation and absorption, and reduction. The reduction process is further divided into wet and dry processes. In the solidification and adsorption processes, concentrated NOx have to be treated in some way or other. The adsorption and wet reduction require at added process cost an oxidizing agent because of the stability of NO. Moreover, the adsorption and absorption necessitate large size equipment and involve difficulties in the regeneration of the adsorbent and absorbent, disposal of secondary products, and abation of secondary pollution with the waste liquids. Thus, with the exception of dry reduction, all processes proposed so far have problems to be solved before they are successfully put to practical use.
The dry process reduction, which reduces NOx to harmless N.sub.2 and H.sub.2 O by a reducing gas mixture containing NH.sub.3, H.sub.2, H.sub.2 S, CO, HC and the like, poses none of the above-mentioned problems and is therefore under development in many segments of industry. One way of accomplishing dry reduction is to effect the reaction of NOx with NH.sub.3 in the reducing gas mixture in the absence of any catalyst at elevated temperature. Disadvantages of this method are the high and limited effective temperature range, large quantities of reducing gases to be discharged, and the necessity of a large reducing gas volume, several times as much as the NOx to be made harmless.
Another method depends on a catalytic reduction reaction with a catalyst for the transformation of NOx into innocuous forms (the process being hereinafter called "denitrification"). The method is classified into two types according to the type of the reducing agent chosen. Of these, the selective catalytic reduction which is not influenced by the presence or absence of O.sub.2 in the exhaust gases to be handled, is believed economically advantageous because the reaction proceeds at a relatively low temperature and with the same amount of a reducing agent as the NOx.
It is an object of the present invention to provide a catalyst useful in the reaction of NOx together with the reducing agent NH.sub.3.
Catalysts for the selective catalytic reduction process are generally known to be effective when comprising: a carrier which may be alumina, titania, zirconia, silica, their mixture, or one of porous substances, such as, diatom earth and zeolite, used either singly or in combination; and a catalytically active ingredient to be chosen from among the oxides and sulfates of transition elements of base metals as well as precious metals. Alumina type carriers are susceptible to the poisoning effects of SOx, and therefore, they may shorten the catalyst life and may sometimes prove uneconomical in treating a large volume of exhaust gases.
Generally, SOx-containing combustion waste gases also contain soot and dust. For these so-called "dirty" gases, the reactors packed with ordinary granular catalysts are of no use because the dust chokes the catalyst bed as the gases pass through it. Of the transition elements of base metals, harmful heavy metal elements, such as, chromium, when incorporated into catalysts, might come off from the supports and cause a problem of secondary environmental pollution.
Attempts to solve these problems have been made. To combat the poisoning by sulfur oxides, durable carriers of titania, zirconia, and the like have been employed. Against the choking effect of the dust, moving-bed reactors in which the granular catalysts are made movable, and catalysts of special shapes, such as, honeycombs and hollow cylinders, have been introduced.
However, there are a number of disadvantages. The material cost of titania and the like is high. The specific gravities of the catalysts are so heavy that the reactors must be reinforced with additional support frames. Complexity in design calls for special technical skill in building the moving-bed reactors. Site limitations render it difficult to add the denitrifying equipment to existing installations. Furthermore, titania and zirconia do not lend themselves readily to forming or molding to desired shapes.
Particularly, because of the relatively large amounts of soot and dust in these gases, e.g., those produced from the combustion of bottom oil and coal as opposed to natural gas or kerosene, clogging of the catalysts can best be avoided by having the carrier be in a form, e.g., sheets, parallel plates, such that the gas contacts the catalyst in a manner so that the particulate soot and dust will not impinge upon and clog the pores of the carrier. It is thus important that the carrier be susceptible to forming into the desired shape and have sufficient strength to maintain the shape during handling and use.